Lecture 9/10 - Plexuses, Motor Neurons, Localizing Neurologic Lesions

Question 1

the brachial plexus and lumbosacral plexus - what is it, what is included, why do spinal nerves coalesce, what gives rise to specific peripheral nerves (+examples)

Answer 1

a pattern of organization of the peripheral nerves that originate from spinal cord levels C6-T2 (Brachial plexus) and L4-S2 (Lumbosacral plexus)

spinal nerves coalesce in a specific manner to allow innervation (sensory and motor) of specific regions/muscles by nerves from more than one spinal level

the spinal nerves from two to four different (but adjacent) spinal levels give rise to specific peripheral nerves; e.g., radial nerve is composed if C7, C8, T1 and the sciatic nerve comprises L6, L7, S1

a single spinal segment may provide axons for more than one peripheral nerve (e.g., the subscapular and median nerves have roots in C8)

Question 2

what spinal nerves give rise to the musculocutaneous nerve (peripheral nerve)

Answer 2

C6, 7, 8

Question 3

what spinal nerves give rise to the radial nerve (peripheral nerve)

Answer 3

C7, 8, T1, T2

Question 4

what spinal nerves give rise to the median nerve (peripheral nerve)

Answer 4

C8, T1, T2

Question 5

what spinal nerves give rise to the femoral nerve (peripheral nerve)

Answer 5

L4, L5, L6

Question 6

what spinal nerves give rise to the obturator nerve (peripheral nerve)

Answer 6

L4, 5, 6

Question 7

what spinal nerves give rise to the sciatic nerve (peripheral nerve)

Answer 7

L6, 7, S1, S2

Question 8

what spinal nerves give rise to the pudendal nerve (peripheral nerve)

Answer 8

S1, 2, 3

Question 9

what is the plexus name for the cervicothoracic enlargement and lumbosacral enlargement

Answer 9

brachial plexus and lumbral plexus

Question 10

how do we clinically distinguish between upper and lower motor neuron damange? (2 things)

Answer 10

1. evaluate the tone in the muscles of the affected limb
2. evaluate the reflexes

Question 11

when evaluating the tone of muscles in the affected limb to determine where the damage is - what would you see with LMN vs UMN damage

Answer 11

LMN damage = leads to DECREASED TONE (flaccid paresis/paralysis) and after a week or so, NEUROGENIC ATROPHY

UMN damage = leads to INCREASED tone (spastic paresis/paralysis). disuse atrophy develops over a longer time period as the animal no longer uses the limb effectively

Question 12

when evaluating the reflexes for UMN vs LMN, would you expect to see exaggerested, normal or diminished

Answer 12

LMN damage - reflex would be diminished

UMN damage - reflex would be exaggerated (loss of inhibitory inputs by UMN in descending tract)

Question 13

algorithm for localizin a lesion in spinal cord vs PNS? questions and explanations

Answer 13

1. does the animal have a neurologic lesion? neurologic exam reveals decreased or loss of function in one or more limbs: proprioceptive abnormalities, abnormalities in gait (ataxia, positioning abnormalities), weakness, decreased of absence in pain sensation
2. what limb(s) are affected? 1 limb only is likely PNS, 2 or 4 lumb affected (bilateral) - likely the spinal cord
3. is there ataxia? presence of ataxia supports the conclusion that the lesion is likely in the spinal cord (shows that proprioceptive tracts are damaged)
4. what reflexes are decreased? if the lesion is in the spinal cord at the cervicothoracic enlargement, reflexes in the forelimb will be DECREASED. if the lesion is in lumbosacral enlargement, hindlimb reflexes will be DECREASED. If the lesion is between C1-C5 or T3-L3, NO DECREASE in reflexes will be observed in the limbs
5. consider again what limbs are affected - hindlimbs only affected = lesion must be caudal to T2. this is where the last spinal nerves for the forelimb enter/exit so the lesion doesnt affect the motor ofr sensory fibers about that level and the forelimbs appear normal. forelimb AND hindlimb affected = lesion must be above T2 because its affecting the sensory and motor tracts pathways in ALL legs

consider 4 and 5 together. if hindlimb ONLY are affected and hindlimb reflexes are affected, lesion is L4-T2. If hindlimbs only are affected and no reflexes are decreased, lesion is T3-L3. if forelimbs and hindlimbs are affected AND forelimb reflexes are decreased, lesion is C6-T2. if forelimbs and hindlimbs are affected and no reflexes are decreased, lesion is C1-C5

Question 14

where is the lesion if hindlimb ONLY are affected and hindlimb reflexes are affected,

Answer 14

L4-S2

Question 14

where is the lesion if hindlimbs only are affected and no reflexes are decreased

Answer 14

lesion is T3-L3.

Question 14

where is the lesion if forelimbs and hindlimbs are affected AND forelimb reflexes are decreased

Answer 14

lesion is C6-T2.

Question 15

where is the lesion if forelimbs and hindlimbs are affected and no reflexes are decreased

Answer 15

lesion is C1-C5